1. Field of the Invention
The present invention concerns a portable electric tool equipped with a device that can be used to precisely determine, continuously and without contact, the relative position between two cooperating parts of two cooperating elements of said tool, at least one of which is linearly or angularly movable. It can be applied to portable electric tools and, in particular, although not exclusively, to electronic pruning shears or similar tools that have cutting blades whose closure is controllable.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Most frequently, in devices that involve the linear or angular travel of a movable element according to a user-defined order, we find two cooperating elements such as, in the case of portable electric tools: a control that gives the order and may consist of a trigger; and an actuator that carries out the action and may be connected to the motor by a transmission system.
The connection between the control and the actuator is electronically provided by a control board having a microprocessor and that must also measure the linear or angular travel of the actuator to ensure that the control order has in fact been carried out.
Currently, there exist a variety of devices that can be used to fulfill this function, having their inherent advantages and, especially, their inherent disadvantages.
For example, we know of systems of potentiometric linear or angular sensors used to measure travel through a change in the value of their resistance, this change being linear and processed by an electronic control board. The principal drawbacks of these potentiometric linear or angular sensors are the presence of two parts in contact, which slide against one another, each of which is affixed to one of the moving parts; the length of these devices being equal in length to the travel (linear or angular), they take up space, and their wear, associated with their friction, results in changing precision over time and a relatively short lifecycle.
To avoid contact between the moving parts, systems exist that make use of Hall-effect sensors C1, C2, C3, C4, C5, mounted successively on the fixed part and a radially magnetized magnet affixed to the moving part, the Hall-effect sensors issuing a signal whenever they are presented with a magnetic field.
The drawbacks of this solution, illustrated in FIG. 1, include the necessity of using a large number of costly sensors and, especially, in that it cannot be used to accurately determine the position of the moving part between two sensors, resulting in low precision. The total travel L being the sum of the travels l1+l2+l3+l4, the precision is thus that of the travel between two sensors. It is impossible to determine a random travel position with precision without having an infinite number of sensors, which is physically impossible.
Systems are also known that make use of a magnetoresistive sensor (a particularity of which is that it measures the inclination of field lines in a magnetic field) together with a radially magnetized magnet (magnetization is perpendicular to the magnet).
This solution, illustrated in FIG. 2, has the advantage of being simple and contact free, but it has the disadvantage of lacking precision and requires a considerable distance between sensor and magnet, especially when travel is significant. In this diagram we find that for a given travel, one can measure, at the end of travel, an angle of 45° of the magnet's field line, the distance between the magnet and the sensor “l” must therefore be L/2. Consequently, as the travel increases, distance “l” increases. Additionally, the greater the distance, the stronger the magnet's magnetic field must be in order to be detected by the sensor.
We also know (DE-19612422, EP-0997706, EP-1074818, FR-2898189) of a device for determining the relative position between two cooperating parts at least one of which is linearly movable, one of these parts being equipped with a magnet whose magnetization is oriented, whereas the other part is equipped with a magneto-resistive sensor, wherein the analog electrical information emitted by the magneto-resistive sensor can be analyzed based on the inclination of the magnetic field lines presented to it in order to determine the relative position between the two parts with precision.
The invention concerns the integration of such devices in portable electric tools and, more specifically, in electric pruning shears or similar cutting tools in which it is advantageous to precisely determine and control, continuously and without the contact of mechanical elements, the direction of movement and the position of at least two cooperating parts of such tools.
In a manner known per se, these electric pruning shears have at least one movable cutting blade mounted at the anterior extremity of a body shaped to serve as a handgrip and within which is lodged an electric gear motor whose output is connected to said movable blade by means of a moving drive part or actuator, and a pivoting trigger accessible from the exterior of the body capable of activating an electrical control device.
It is known that into such tools can be integrated devices for controlling, with more or less precision, the position of the blade actuator and, consequently, that of said blade, when the latter is opened and closed, based on the control order given by the user by means of the trigger, so that the movement of said blade is dependent on the movement of said trigger, with a concern for the precision of the cut, comfort, and safety.
In FR-2614568 a description is given of a servocontrolled portable electric tool in position wherein the moving part driving the pivoting blade consists of the ball assembly of a ball bearing ball screw system that can be driven axially in one direction or the other depending on the direction of rotation of the screw, wherein the pivoting trigger used to actuate the electric control device is slaved to said ball whose linear displacements it consequently follows. These rectilinear trajectories of the trigger limit the ball's length of travel to the length of the user's finger. Moreover, because the electric control device moves together with the ball, it is necessary to link said assembly and the connector by means of a flexible link or flexible circuit, such that wear on this flexible circuit may lead to malfunctions.
In FR-2770372 different embodiments of a mechanically activated portable cutting tool are described, wherein the position of the movable cutting blade is detected by various devices that make use of one or more potentiometers, some linear and some rotary, or even through the use of a mixed linear and rotary potentiometer device. We note that in all these cases each potentiometer must be linked, on the one hand, to a moving part and, on the other hand, to the fixed body of the tool by means of oblong grooves for proper operation: in order to transform certain rotational movements into linear movements and due to the fact that the centers of rotation are not necessarily the same for the moving part and the potentiometer in the case of rotary potentiometers. The principal drawback of these devices is that the presence of functional play at the oblong grooves is essential and, consequently, this play necessarily introduces an error in the value of the position transmitted by the potentiometers, this error increasing with operational wear. Other drawbacks follow from the fact that the potentiometers are components that have two tracks in permanent contact that wear quickly if a large number of maneuvers are carried out; additionally, such components are costly and bulky.
WO-2005/084416 describes a mechanically activated cutting tool in which the movable cutting blade is detected by a plurality of Hall-effect sensors distributed along the arc of a circle on a fixed element of the tool body so as to enable the detection of the angular movements of a magnet affixed to an element integral with said pivoting cutting blade. The disadvantage of this device is that it cannot determine with high precision the position of the pivoting cutting blade (the precision is equal to the angle between the two sensors) and is expensive given the need to use a plurality of costly Hall-effect sensors.
Another object of the invention is to provide users with electric pruning shears that are free from the previously described drawbacks of known electric cutting tools and that can be used to provide greater precision and speed during cutting activities, as well as greater safety for users, by very precisely linking the open position of the blade to the position of the trigger controlled by the user.